Tuning lipid mixtures to induce or suppress domain formation across leaflets of unsupported asymmetric bilayers

Abstract

Plasma membranes of cells are asymmetric in both lipid and protein composition. The mechanism by which proteins on both sides of the membrane colocalize during signaling events is unknown but may be due to the induction of inner leaflet domains by the outer leaflet. Here we show that liquid domains form in asymmetric Montal-Mueller planar bilayers in which one leaflet's composition would phase-separate in a symmetric bilayer and the other's would not. Equally important, by tuning the lipid composition of the second leaflet, we are able to suppress domains in the first leaflet. When domains are present in asymmetric membranes, each leaflet contains regions of three distinct lipid compositions, implying strong interleaflet interactions. Our results show that mechanisms of domain induction between the outer and inner leaflets of cell plasma membranes do not necessarily require the participation of membrane proteins. Based on these findings, we suggest mechanisms by which cells could actively regulate protein function by modulating local lipid composition or interleaflet interactions.cholesterol ͉ induction ͉ raft ͉ membrane ͉ phase C ell membranes are asymmetric in lipid composition between the inner and outer leaflet (1). Lipids of the two leaflets are assumed to also differ in their ability to separate into domains enriched in particular lipids and proteins. Model membranes composed with the goal to mimic the lipid mixture of the outer leaflet of a cell membrane separate into two liquid phases (2-4), whereas at least one inner leaflet mixture does not (5). To explain colocalization of inner and outer leaflet proteins during signaling events, it has been hypothesized that domains in the outer leaflet induce domains in the inner leaflet (6-8). However, domain induction across membrane leaflets is controversial. Some researchers argue that only proteins should be able to induce domains across leaflets (9). Other groups, including our own, have observed what seems to be domain induction across leaflets of protein-free membranes deposited on solid surfaces, but generally these efforts have been frustrated by experimental difficulties (7, 10-12).Here we construct asymmetric, protein-free, planar bilayers in water. One leaflet's composition would phase-separate in a symmetric bilayer and the other's would not. We show that liquid domains form in both leaflets of the asymmetric bilayer. One leaflet induces phase separation in the other. Equally important, we show that changing the lipid composition of the second leaflet suppresses domain formation in the original leaflet. We find that leaflets are strongly coupled. Our results imply that cells could tune membrane composition to create or annihilate domains. Because induction of domains occurs in planar membranes devoid of membrane proteins, induction of domains in cell membranes need not rely on membrane curvature or protein coupling (9,13,14).We construct membranes of diphytanoylphosphatidylcholine (DiPhyPC), dipalmitoylphosphatidylcholine (DPPC), and cholesterol (Chol)...

Lipid Domain Co-localization Induced by Membrane Undulations

“…12 is symmetric under the transformation k 0 /k dom ; k dom /k 0 , L fluct;free z0:018 k B T=nm 2 also for the case of softer Ld domains embedded within the more rigid Lo phase. Thus, membrane undulations also promote co-localization of the softer Ld domains, in agreement with experimental observations (31,32). Such a symmetry is absent when the domains are far apart (17).…”

Membrane heterogeneity – from lipid domains to curvature effects

“…Furthermore, the binary picture of the cell membrane divided in raft and non-raft regions is oversimplified: Not only that the membrane contains significantly more than 3 different types of lipids which could result in several liquid phases all with different properties. Even with only 3 components, membranes that are asymmetric -the compositions of the two leaflets are differentexhibit 3 distinguishable phases (21). Also the idea that rafts are stable entities was challenged (19).…”

Membrane heterogeneity: Manifestation of a curvature-induced microemulsion

“…The small size of the domains could then be attributed to the effects of the cytoskeleton 8 . One difficulty with this hypothesis is that it is known that a bilayer of composition which mimics the inner leaf of the plasma membrane does not undergo phase separation 9 , and that the coupling of such a leaf to another which does tend to phase separate produces a bilayer in which the miscibility transition either occurs at a greatly reduced temperature or is eliminated entirely 10,11 .…”